Among prior optical systems using a reflection optical system to obtain full 360°-direction (full-panoramic) images, there is one made up of a front unit comprising a transparent medium that is rotationally symmetric about a center axis and includes two internal reflecting surfaces and two transmitting surfaces and a rear unit that is rotationally symmetric about the center axis and has positive power, as set forth in patent publications 1 and 2.
Patent Publication 1    U.S. Pat. No. 4,566,763
Patent Publication 2    U.S. Pat. No. 5,473,474
A problem with either of patent publications 1 and 2 is, however, that when full-panoramic images are picked up with the optical system directing to the zenith, much harmful flare light coming from the zenithal direction causes image quality to go worse, because the effective diameter of the first transmitting surface grows large as a consequence of an entrance pupil within a section including a center axis being located the center axis.